It is, therefore, necessary to implement at the power supply output a protection device to prevent the fault current from increasing so that it cannot reach the limit where the sum of the load and fault currents is equal to the maximum current which the power supply can deliver.
In general, power supplies which supply a power exceeding 20A use a circuit breaker tripped at a fixed threshold.
This solution cannot be implemented in a machine where the load to be supplied is not always the same and depends on the user requirements. For example, in a teleprocessing system including a communication controller of an architecture type such as illustrated in FIG. 1 of the European patent application, published under number 0005722, a power supply unit is used for one or more line adapters. Therefore, depending on the network configuration, that is, on the number of terminals, the power supply load varies in a proportion of 1 to 4. Under these circumstances, not all network configurations can be protected satisfactorily.
This problem can be solved by measuring the load current at machine installation time and by adjusting the tripping point of the circuit breaker manually. However, this complicates the installation steps and produces a source of additional human errors so that it is hardly satisfactory.
The detection thresholds can also be adjusted automatically by program, as described in the IBM Technical Disclosure Bulletin, Vol. 20, No. 9, Feb. 1978, page 3589. It is obvious that this possibility can only be used in machines including microprocessors; it also complicates the steps required to make these machines operational because in addition to the operations generally required for this, it is necessary to choose the limiting values and to load these into storage, which requires the intervention of a field engineer.